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Full text of "Handbook Of Chemical Engineering - I"

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278
CHEMICAL ENGINEERING
ABBREVIATED SCREENING TEST
Screen mesh	Per cent by weight feed	Per cent by weight plug product	Per cent by weight overflow
.     On   14	26.9	38.5	
20	10.7	15.2	
28	9.4	13.0	
35	7.2	9.8	
48	6.4	8.5	
65	3.9	5.0	0.25
100	4.0	4.4	2.75
150	3.3	2.4	5.00
200	0.4	1.0	3.50
Through 200	27.8	2.2	88.50
In the second use some of the material was too coarse to pass a 6-mesh screen. The large capacity is due to the coarseness of the feed. On material which will pass meshes ranging from 20 to 60 the rate of feed will range from 50 to 100 tons per 24 hr.
Thickeners.—These devices in point of fineness of material to be separated come below the classifiers. They are capable of giving clear overflows. With these devices liquids containing as low as 1 per cent solids can be thickened to a discharge containing as high as 40 per cent solids. With favorable factors the amount of solids in the discharge may be as high as 75 per cent.
The slope of the bottom of decantation tanks employing gravity discharge of the solids solely, ranges all the way from 0° to the 60° used in ore slime and paper pulp recovery work: Even with the extreme slope of 60° used in ore work there is an accumulation on the side of the tank by contact action which may reach a thickness of half an inch. Where the subsiding solids remain in the bottom of the tank as a suspense after slow settlement, a flat bottomed or nearly flat-bottomed tank may be used. Many organic compounds settle only to this degree particularly if they are in more or less of a colloidal state and if the percentage of solids is small it will be best to allow them to accumulate as long as possible so as to get the greatest compacting effect. If they are then in a condition to flow out freely a flat-bottomed or nearly flat-bottomed tank may be employed and the sludge can be discharged by a simple pipe and valve. If the solids do not flow out freely they can be removed periodically with hose and shovel, the tanks being provided with a system of drain cocks for removing the clear supernatant liquid.
The Booth water-softening apparatus for softening water by the soda-lime method employs a flat bottomed tank. The discharging means consists of one or two headers laying in the bottom of the tank and connected to a valve outside. Each header has a number of stub pipes screwed in at right angles on each side and open at their outer ends, the object being to uniformly reach, in discharging, all parts of the accumulated sludge.
Callow Tanks.—These tanks are illustrated in Figs. 9 and 10. For separation, where a clean liquid overflow is desired, the use of the smaller sizes shown in Fig. 9 is not to be recommended. As an inspection of the capacity table accompanying Fig. 9 will show the tanks have a small capacity and are suitable to small operations. Used in batteries they occupy more space than a single tank of equal settling area and capacity and there is the extra annoyance and expense in insuring the free running of the discharges. A battery of 50 8-ft. tanks requires not